The invention relates to a plate spring of a substantially circular-ring-shaped form having a substantially externally conical upper side and a substantially internally conical underside between which there are positioned an outer annular edge and an inner annular edge, and to a method of producing plate springs. Plate springs, while having an extremely short length, usually have very steep spring rates, i.e. they combine high returning forces with a short spring travel. They are frequently used in the form of spring packages with identical or changing cone positions, but also in the form of individual spring elements, e.g. as compressive coupling springs in motor vehicles or in the form of piston returning springs in automatic gearboxes.
The concept of introducing inherent compressive stresses and compressive pre-stresses to increase the fatigue strength of plate springs is generally known. In industrial practice, this is normally achieved by shot-blasting the plate springs. For this purpose, there is available a number of different shot-blasting methods such as centrifugal blasting (drum blasting) or injector blasting (shot-blasting by air jets). The former method is particularly suitable for treating bulk material, i.e. plate springs with smaller diameters of just a few mm, whereas the latter method is suitable for larger individual parts, for example plate springs with a diameter of several 100 mm. The objective of such treatments is to achieve as uniform a compaction as possible of all surfaces. This results in uniform internal compressive pre-stresses in all surface layers of the component, with “surface layer” referring to the component surface layer subjected to the compaction process.
As is generally known, plate springs of the above-mentioned type experience a settlement behavior, i.e. a relaxation-related decrease in force as a function of time or as a function of the number of load cycles. To be able to maintain the required characteristic curves for the entire service life, the plate springs first have to be built in with an excess of pre-stress, which, initially, leads to an excessively hard operation of the functional parts in question or, it is necessary to provide a readjustment facility for their stops, which increases the costs of the design in question.